Sprite on mouse-button hold

[Clunk_Head]

I'm working with pygame 1.9.6
I am trying to make a program that displays a translucent sprite only when the left mouse-button is held down.
This sprite is supposed to track with the mouse position, show the background through it, and disappear when the mouse-button is released.

Currently:
It is tracking with the mouse position.
It is stamping a new copy instead of just moving the sprite.
The sprite(s) are not disappearing when the mouse-button is released.
The sprite alpha is not working, they are a solid color.

Here is the complete code. It is relatively short.

import pygame
import sys
from pygame.locals import *

pygame.init()

LTGRAY = (199, 210, 199)
HLIGHT = ( 80, 100,   0)
BLACK  = (  0,   0,   0)

main_surface = pygame.display.set_mode((900, 650), 0, 32)
main_surface.fill(LTGRAY)

clock = pygame.time.Clock()
fps = 30

pygame.draw.line(main_surface, BLACK,
                     (0, 0), (900, 650), 3)
pygame.draw.line(main_surface, BLACK,
                     (900, 0), (0, 650), 3)

class hlight(pygame.sprite.Sprite):
    def __init__(self):
        pygame.sprite.Sprite.__init__(self)
        self.image = pygame.Surface((50, 50), flags=pygame.SRCALPHA)
        self.image.fill(HLIGHT)
        self.image.set_alpha(35)
        self.rect = self.image.get_rect()
        
    def update(self, pos):
        self.rect.x, self.rect.y = pos
        
highlights = pygame.sprite.Group()

while True:
    for event in pygame.event.get():
        if event.type == QUIT:
            pygame.quit()
            sys.exit()
        elif pygame.mouse.get_pressed()[0]:
            if len(highlights) == 0:
                highlights.add(hlight())
        elif event.type == pygame.MOUSEBUTTONUP:
            for highlight in highlights:
                highlight.kill()
        
    highlights.update(pygame.mouse.get_pos())
    highlights.draw(main_surface)
    pygame.display.update() 
    
    clock.tick(fps)

Thanks for any help